Automatic telephone system



Nov. 4 1924: 1,513,777

B. D. WILLIS AUTOMATIC TELEPHONE SYSTEM Original Filed Sept. 22. 1920 3Sheets-Sheet 1 7 HM- 1 \1 i b I:

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Nov. 4 1924. 1,513.777

B. D. WILLIS AUTOMATIC TELEPHONE SYSTEM Original Filed Sept. 22. 1920 I5Sheets-Sheet 5 Patented Nov. 4, 1924.

UNITED STATES PATENT OFFICE.

BERNARD DARWIN WILLIS. OF OAK PARK, ILLINOIS, ASSIGNOR TO AUTOMATICELECTRIC COMPANY, OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS.

AUTOMATIC TELEPHONE SYSTEM.

Application filed September 22, 1820, Serial No. 412,070. Renewed May16, 1924.

To all whom it may concern:

Be it known that I, BERNARD D. WiLLIs, a citizen of the United States ofAmerica, and a resident of Oak Park, Cook County, and State of Illinois,have invented certain new and useful Improvements in Automatic TelephoneSystems, of which the following is a specification.

My invention relates in general to automatic telephone systems, but moreespecially to private automatic systems of the general type which is incommon use for furnishing telephone service to large factories. businesshouses, etc., and the object of the invention may be broadly stated tobe the provision of improved trunk circuits for connecting a privateautomatic exchange of this character with a main exchange.

A special feature in this connection, which is not limited, however, touse with a private automatic exchange, is the provision of novel andeffective means for making an interoflice trunk line busy to switches inthe private automatic exchange when the trunk line is taken for use atthe main exchange.

There are other features which, together with the above, will be fullydescribed hereinafter reference being had to the accompanying drawings.

I have chosen to illustrate my invention in connection with a privateautomatic exchange located in a territory served by a manual publicexchange and connected with such public exchange by trunk lines. In thedrawings, comprising Figs. 1, 2, and 3, I have shown by means of theusual circuit diagrams sufficient of the apparatus in the manualexchange and in a private automatic exchange embodying the principles ofmy invention to enable the same to be explained and understood.

Fig. 1 shows the manual equipment, which is all old and is includedbecause of its cooperation with the rest of the equipment. Substation Ais an ordinary manual telephone substation having the usual lineequipment at the exchange. The reference character 0 indicates anoperators cord circuit of a type which is in common use. Conductors 36and 37 are conductors of one of the trunk lines extending to the privateautomatic exchange under consideration. This trunk line has lineequipment at the manual switchboard similar to the line equipmentprovided for ordinary subscribers lines.

The trunk lines connecting the manual exchange with the privateautomatic exchange pass through an attendant operators position,commonly referred to as an attendants cabinet, before they terminate inthe incoming connector switches which have access to the local lines.The apparatus at the attendants cabinet is associated with the trunkline comprising conductors 36 and 37 and is shown in Fig. 2, beingindicated therein by the reference character M. The operation of thisapparatus will be fully explained hereinafter.

The incoming connector in which the trunk lines illustrated terminatesis shown in Fig. 3 and is indicated by the reference character H. Thisconnector switch is of the well known Strowger vertical and rotary type,and its mechanical construction therefore will need no explanation. Theoperation of the circuits will be described. The connector H in commonwith other similar connectors has access to the local lines in theprivate automatic exchange, one of which is the line extending to thesubstation A. This substation is an automatic telephone substationsimilar to the substation A, except that it is provided with a callingdevice S for controlling the automatic switches which constitute theprivate automatic exchange switchboard.

The local subscribers lines are provided with individual line switches,the line switch associated with the line of substation A being indicatedby reference character C. Theseline switches are of the Well knownrotary type, in which the wipers have no normal position and move in aforward direction only, and have access to a group of local connectors.The latter switches are of the type commonly known as selectorconnectors, such as is shown in the U. S. patent to Dcakin, No.1,164,479, granted December 1 1, 1915, and have access not only to locallines but also to the trunk lines connecting with the main exchange.

Having described the apparatus briefly I will now proceed to point outmore in detail the novel features of my invention and explain theoperation thereof. For

this purpose it will be assumed that the subscriber at substation A, inthe manual public exchange, desires to secure a connection with thesubscriber at substation A, who is connected to the private automaticexchange. When the receiver is removed at substation A a circuit iscompleted over the line conductors 11 and 12 for the l ne relay 2, whichoperates to light the line lamp L. When the operator at whose positionthe lamp appears perceives the signal.

she will answer the call by inserting the answerin plug of an idle cordcircuit, cord circuit 6 for example, in the answering jack J. As aresult the cut oil relay 3 is energized, the line relay 2 is deenerized, and the line lamp L is put out I ey K being thrown to listeningposit on, the operator may now talk to the calling subscriber, and onfinding that he desires to be connected with a subscriber in the privateautomatic exchange, she will insert the calling plug of the cord circuitin use in the jack of an idle trunk line extending to such privateautomatic exchange. Assuming that the trunk line shown in the drawing isthe one taken for use, when the'plug is inserted in jack J a circuit iscompleted for the cut off relay 30 in series w th sleeve relay 17 andthe former relay is energized to cut off the line relay 31.

Before considering the operations which take place at the privateautomatic exchange responsive to the insertion of the plug n jack J itwill be necessary toexplain briefly the normal circuit conditions in thetrunk line. A repeating coil is inserted in the trunk line at theattendants cabinet and it will be observed that a polar relay 52 isconnected around the terminals of the condenser which joins the two lefthand windings of this repeating coil. Thus relay 52 may be said to bebridged across the trunk conductors 36 and 37 At the main exchange theline relay 31 is n orma ll bridged across the trunk line in series witthe battery and it follows therefore that the polar relay 52 will benormally energized, the current flow in its winding being in such adirection that its armature is held in position as shown in the drawing.Due to the fact that the high resistance 40 (about 12,000 ohms) isincluded in the circuit of the polar relay, the line relay 31 at themain exchange is not operated under normal conditions.

It will be seen that the upper trunk conduct-or 36 connects to batteryat the line relay while the lower trunk conductor 37 connects to ground.When the plug is inserted in jack J as described hereinbefore,conditions are reversed and the upper trunk conductor 36 will extend toground through the upper winding of the supervisory rela 16 while thelower trunk conductor 37 wi go to battery by way of the lower winding ofthe same relay. It follows, therefore,

that when the plug is inserted in the jack the direction of current flowover the trunk line will be reversed, and as a result the polar relay 52will operate its armature 61' so as to close its working contact. .Thiscloses a circuit for the slow acting relay 54 which upon energizingprepares a circuit for relay 53 at its armature 65, connects ground tothe release trunk conductor 78 at its armature 64 (in order to make thetrunk line busy to the local selector connectors), and at its armature66 completes a control circuit extending over conductor 7 5 to the linerelay 100 of the connector H, Fig. 3. It is noted at this point thatalthough a circuit is completed for relay 53 at armature 65 of rela 54,the said relay 53 is not operated at t is time, because its winding isshort circuited over a path which extends by way of armature 63 to thegrounded armature 61 of the polar relay 52.

At the operators cord circuit the supervisory relay 16 is not energizeddue to the inclusion of the high resistance 40 in the trunk circuit, andthe supervisory lamp L will be lighted. The other supervisory lamp L isof course extinguished due to the fact that the double wound relay 15 isenergized over the calling subscribers line circuit.

The manual operator will now throw her key K to ringing position inorder to signal the attendant at the private automatic exchange. By thisoperation ringing current is transmitted over the trunk line, causingthe polar relay 52 to vibrate its armature, whereupon the short circuitis intermittently removed from around the winding of relay 53 and thesaid relay 53 is energized. Up'on energizing, relay 53 breaks theconnection to the contact of the polar relay 52 at armature 63 therebyinsuring that the short circuit will not be reestablished. Relay 53 alsocloses a circuit for the answering lamp L at its armature 62. When theattendant sees the lamp L lighted, she will answer the call by throwingher listening key K thereby connecting her head set across the trunkline. The operation of key K also closes a circuit for the relay 51which upon energizing locks itself at its armature 59, the lockingcircuit including the winding of relay 53. Relay 51 also prepares acircuit for the supervisory lamp L at its armature 58, and at itsarmature 60 short circuits the resistance 40. When the resistance 40 isshort circuited relay 16 in the operators cord circuit at the mainexchange is energized and the supervisory lamp L is extinguished, thusnotifying the operator that the attendant ill ber of the desiredstation, and the latter will;

complete the connection as follows:

The calling device key K is first operated in order to cut the callingdevice in on the control circuit extending to the line relay 100, afterwhich the dial may be operated in accordance with the first digit in thelocal number of the substation A. The slfow acting release relay 101being now in energized position, the resulting deenergizations of theline relay serve to transmit a series of im ulses to thevertical.,magnet 107 over the fol owing path: ground at G armature 112and its resting contact, armature 113 and its working contact, oifnormal springs 132 and 134, wiudin of the slow acting series relay 102,and tie winding of the vertical magnet 107 to battery. By the operationof the vertical magnet the wipers 140-142, inclusive, are raised step bystep and are brought to rest opposite the level of bank contacts inwhich the line of substation A is terminated. The slow acting relay 102is energized in series with the vertical magnet, and retains itsarmature 114 attracted throughout the vertical movement of the switch,thereby maintaining the circuit of the vertical magnet intact after theoff normal springs have been shifted, which occurs at the first verticalstep. At the, end of the vertical movement relay 102 falls back andtransfers the operating circuit to the rotary magnet 108.

The attendant will now operate her calling device in accordance with thelinal digit in the desired number, and the line relay 100 is deengerizeda plurality of times as before, this time, ho\vever,'sending impulses tothe rotary magnet 108 as follows: ground (i armature 112 and its restingcontact, armature 113 and its working contact, oti' normal springs 132and 133, armature 114 and its resting contact, winding of the slowacting series relay 106, armature 125 and its resting contact, restingcontact of armature 120 and the said armature, and the winding of therotary magnet 108 to battery. By the operation of the rotary magnet thewipers 140142, inclusive, are rotated over the bank level opposite whichthey have been raised and are finally brought to rest in engagement withthe particular set of contacts in which the line of substation A isterminated, these contacts being indicated in the drawings by referencecharacters 143-145,

- inclusive. The slow acting relay 106 is energized in series with therotary magnet 108 and malntalns its armatures attracted throughout therotary movement of the switch. At its armature 127 relay 106 connectsthe test Wiper 141 with the test relay 1'04 and at its armature 128relay 106 closes a shunt around the break contacts at armature 120 ofthe said test relay in order to guard against the possibility of havingthe rotary magnet circuit opened. while the test wiper is passing overbusy test contacts during the rotation of the switch.

Assuming now that the line of substation A is busy when called, when thetest wiper 141 comes to rest in engagement with test contact 144 it willfind a ground potential signal to the calling subsz-riber and to eitheror both of the operators provided they are across the trunk line. Thisis the usual signal which is transmitted to the calling party to notifyhim that the called line is busy.

Having explained briefly the operation which takes place when the calledline is busy it will be assumed now that the called line is idle. Underthese circumstances, and assuming that the line is connected with asbefore described, when the test wiper 141 comes into engagement withtest contact 144 it will find no ground potential and the test relay 104will not be energized. Then when the slow acting relay 106 talls back acircuit will be completed for the switching relay 105 as follows: fromground at G by way of the working contact of armature 115 and the saidarmature, armature 119 and its resting contact, lower winding of theswitching relay 105, resting contact of armature 127 and the saidarmature, test wiper 141, test contact 144, winding of the switchingrelay 148 of the line switch C, and the winding of the stepping magnet150 to battery. Upon the closure of the above circuit relays 105 and 148are energized in series and the latter relay clears the called line ofits normal battery and ground connections in the line switch (1. Due toa mechanically interlocking device controlled by the armature of theline relay 141-9, relay 148 operates its armature, or armatures, onlyabout half way and the wipers of the line switch are not connected.

In the connector H when the switching relay 105 energizes it establishesa locking circuit for itself at its armature 124, opens the rotarymagnet circuit at armature 125, and at armatures 122 and 126 connects upthe line wipers 140 and 142. Ringing current from the generator Gen isnow periodically projected out over the called line to operate thebridged ringer at substation A, in order to notify the subscriberthereat that he is being called. It is noted that the return path forthe ringing current includes the upper winding of the ring cut off relay103, and when the called subscriber responds ofi relay will be operated.Upon energizing, relay 103 looks itself at armature 117, breaks the rining circuit at armatures 116 and 118, an at the working contacts ofthese same armatures finally completes the talkin circuit.

aving described the operation of the connector H in completing theconnection it will be convenient now to return to the equipment at theattendants cabinet and consider the operations whlch take place there.It is understood of course that the calling device ke K is left inoperated position only while the attendant is using the calling deviceto operate the connector H. When the last digit is called therefore, keyK is restored and the line relay 100 is maintained energized over theoriginal circuit which was established at armature 66 of relay 54. Whenkey K is restored a circuit is completed for the supervisory lamp L asfollows: ground at G, off normal springs 131 and 130, conductor 76, up-

; per contacts of key K working contact of armature 58 and the saidarmature, normally closed contacts of key K and through the lamp L tobattery. The supervisory lamp L remains lighted until the calledsubscriber answers. When this occurs the battery feed relay 50 isenergized and breaks the locking circuit of relay 51 at armature 56.Relay 51 accordingly deenergizes (listening key K having been restoredby this time) and breaks the circuit of the supervisory lamp L, thusputtlng out the light and notifying the attendant that the local partyhas answered. Relay 51 also removes the short circuit around resistance40, but this operation is of no effect because the said resistance isshort circuited at armature 57 of relay 50.

The circuit over which conversation is held is of the usual character,as will be observed, and will need no explanation. When the subscribersare through talking they will hang up their receivers. When thesubscriber at substation A replaces his receiver the supervisory relay15 in the cord circuit '0 will fall back and light the supervisory lampL. When the subscriber at substation A hangs up his receiver the batteryfeed relay 50 will deenergize and remove the short from around the highresistance 40, which is included in the circuit of the supervisoryrelay- 16. Relay 16 is accordingly deenergized and lights thesupervisory lamp L Responsive to the disconnect signals themanualyoperator Wlll pull down the connection. When the plug iswithdrawn from jack J the cut ofl' relay '30 is deenergized and the-linerelay is again connected in bridge of the trunk conductors 36 and 37. Bythis operation the direction of current flow over the trunk and of thepolar relay 52 is again reversed, t at is, the current flow in the trunkcircuit is now in normal condition, and the polar relay 52 isaccordingly operated to move its armature 61 to the position shown inthe drawing, so that its make contact is broken. Relays 54 and 53 arenow deenergized, and the former relay breaks the control circuitextendin to the line relay 100 of the connector The line and releaserelays 100 and 101 are now dcenergized, the latter relay breaks thelooking circuit of relays 103 and 105, and relays 100 and 101 jointlyclose a circuit for the release magnet 109. By the operation. oftherelease magnet the connector H is restored to normal position in theusual manner and is thus made ready for use in setting up anotherconnection.

Having described the process of setting up a main to branch exchangeconnection, it will be convenient now to explain briefly the process ofsetting up a connection in the opposite direction. It will be assumedthat the subscriber at substation A desires to call the subscriber atsubstation A. Since there is nothing novel in the operation of the localswitches in the private and automatic exchange the operation oftheseswitches in connecting with an interoifice trunk line need not eexplained in detail, and it has not been considered necessary to showthe circuit of the local selector connector, which as before stated maybe similar to the selec tor connector shown in the patent to Deakin.When the receiver is removed at substation A the line switch C operatesautomatically to extend the associated line into connection with a trunkline leading to an idle selector connector, and the line and releaserelays in the selector connector which is taken for use are thenenergized in the usual manner in order to prepare the switch for 0eration under control of the calling device' S at the calling station.Assuming that the main oflice trunk lines are terminated in the tenthlevel of the selector connectors, as is the usual practice, the callingdevice will now have to be operated in accordance with the digit 0. Bythis means the wipers of the selector connector are raised step by stepto the tenth level, after which the automatic trunk selecting movementis initiated and the Wipers are rotated into engagement with the contactset in which the first idle trunk line is terminated. It will be assumedfor convenience that the trunk line shown in the drawing is the firstidle trunk line and the wipers of the selector connector will thereforeestablish connection with bank contacts -82, inclusive, Fig. 2. When theconnection is established a ground potential is placed upon the testwiper of the selector connector in the usual manner and a circuit iscompleted byway of the said test wiper,

test contact 81, release trunk conductor 78, armature 64 and its restingcontact, and the winding of relay 55 to battery. Upon energizing, relay55 establishes a locking circuit or itself at armature 68, disconnectsthe section of the trunk line extending to the connector H by means ofarmatures 67, and 71, and at armature 69 prepares a circuit for relay54. The selector connector having finished its operation the ailing lineis extended through to the trun conductors 77 and 79, and the batteryfeed relay 50 is accordingly energized over the line circuit. Relay 50now completes a circuit for relay 54 at its armature 56, and the latterrelay, upon energizing, connects ground to the release trunk conductor 78. If the selector connectors have the precise circuit arrangement shownin the Deakin atent this latter operation is of no particu ar utility,for the conductor 78 will already have been grounded at the test wiperof the selector connector in use. Selector connectors of a later type,however, or some of them at least, place only a temporary ground on thetest wiper, and require a holding circuit to be established in order tomaintain them in operated position. The circuit arrangement shownprovides for the use of a switch of this character if desired.

The connection with the trunk line having been established, it remainsto be seen how the operator at the distant exchange is signalled. Thisis accomplished by the short circuiting of the resistance 40 at armature57 of the battery feed relay 50. When resistance 40 is short oircuited,the line relay 31 which is bridged across the trunk line at the mainexchange is energized and lights the line lamp L Upon perceiving thesignal the operator will answer the call by plugging into the jack J andwill then extend the connection to any desired party, the subscriber atsubstation A, for example, in the usual and well understood manner.

When the plug is inserted in jack J B the direction of current flow overthe trunk circuit and through the winding of the polar relay 52 is ofcourse reversed, as explained in the previous case, and. the. polarrelay is operated to close its make contact. This, however, is of noeflect at the present time, since relay 54 has already been energized.The circuits which control the supervision are the same as have beenalready described. When the subscriber at substation A hangs up hisreceiver the supervisory lamp L is lighted by the deenergization ofrelay 15. When the subscriber at substation A hangs up his receiverrelay 50 deenergizes and connects resistance 40 in series with relay 16,which thereupon deenergizes and lights the supervisory lamp L Theoperator will then pull down the connection. In case the selectorconnector in use is provided with a series holding relay as is theDeakin selector connector, it will release as soon as the callingsubscriber hangs up his receiver, but in case the selector connector isof the type which is held up over the release trunk ahead, it will notbe released until the operator at the main exchange pulls down theconnection. When this occurs the polar relay 52 of course returns tonormal position, relay 54 deenergizes, and ground is removed from therelease trunk conductor 78.

The foregoing describes the operation of the system in the daytime.After business hours the attendant will not be present at her positionand naturally cannot be relied upon to complete incoming calls. It isnecessary therefore to provide other means for completing suchconnections, and in order to take care of; this situation each trunkline is connected through to some particular local station. Each trunkline thus becomes in effect an individual line connecting the associatedlocal station with the main exchange. The settin up of these nightconnections will now fie explained. Assuming that the trunk line shownin the drawings is to be connected through to the line of substation Afor night service, the attendant will actuate the night key K' and willleave the same in operated position. When the key is operated lamps Land L are disconnected, ground is connected to the make contact atarmature 56 of relay 50, thus preparing a circuit for relay 54, andground is connected through a resistance to conductor 75, therebycompleting the control circuit of line relay 100 of the connector H.Since this control circuit remains closed all night the resistance isincluded in the circuit in order to reduce the flow of current andprevent the relay from overheating. The attendant will now throw hercalling device key K to connect the calling device S into the controlcircuit of the line relay 100, after which the calling device may beoperated to control the connector H for the purpose of connecting thetrunk line with the line of substation A. The connector operates in theusual manner and when the subscriber answers he is informed that histelephone is being connected for night service. The operator will nowrestore the calling device key and her listening key. Having extendedthe other trunk lines through to their respective night service stationsin the same manner the attendant may now leave her position.

The operation of putting through calls when the night serviceconnections are up will doubtless be obvious but it may be well todescribe the same briefly. Considering first, incoming calls from themain exchange, itwill be clear that when the manual operator plugs intoa trunk line and a circuit for relay 54, which pulls up and placesgroundon the test contact 81 and the multiplesthereof, in order to makethe trunk line busy to the selector connectors in case one of theseshould be operated. In

' case a party whose telephone is connected for night service desires tocall a main exchange subscriber he may do so simply by lifting hisreceiver from the hook. This operation, "supposing the subscriber atsubstation A' to be calling, energizes relay and short circuits theresistance 40, thereby operating the line relay 31 to light the linelamp L Relay 50 of course operates relay 54 now as in the previous casein order to ground the test contacts in the selector connector banks andmake the trunk line busy. In case anyone should wish to use one of thetelephones which is not connected for night service, he may do so ofcourse, and can call the main exchange as described hereinbefore, justthe same as though the night service connections were not up.

It will be seen that I have devised a novel and highly eflicient trunkcircuit for connecting a private automatic exchange with a mainexchange, including features which will doubtless be found to be ofutility in systems difi'ering widely from the particular system shownand described herein. A feature of considerable importance consists inthe provision of a polar relay which is normally energized in order tomaintain the trunk line in idle condition and accessible to the selectorconnectors. Since this ,relay must remain energized in order to same asthough it had been taken for use at the main exchange. This feature isvery advantageous therefore and is obviously applicable not only totrunk circuits connecting a private automatic exchange with a manualexchange but also to trunk circuits connecting a private automaticexchange with another automatic exchange.

For these reasons, and because of other features which also have a widerange of utility, I do not wish to ,be restricted to the precise form ofmy invention which is shown and described herein, but desire rather toinclude and have protected by Letters Patent all forms of my inventionwhich may hereinafter be devised, limited onl by the scope of theappended claims. a at I claim 'as myinvention is: 1. In a telephonesystem, a trunk line,

means for associating a test potential with said trunk to make the samebusy, and a relay permanently bridged across said trunk and normallyenergized over said ti'unk'to prevent the application of said testpotential.

2. In a telephone system, a trunk line and test terminal therefor, meansfor placing a potential on said terminal to make the trunk line busy, apolar relay energized over the trunk to prevent the application of saidpotential to said terminal, and means for reversing the direction ofcurrent flow over said trunk to deenergize said relay.

3. In a telephone system, a trunk line and test terminal therefor, meansfor placing a potential on said terminal to make the trunk line busy, apolar relay energized over the two sides for the trunk in series toprevent the application of said potential to said terminal, and operatorcontrolled means for reversing the direction of current fiow over saidtrunk to deenergize "said relay.

4. In a telephone system, a twp way trunk line, a relay at one end ofsaid trunk and normally energized thereover to maintain the trunk inidle condition, means efi'ective when the trunk is taken for use at theother end for deenergizing said relay, and a test circuit completed bysaid relay to make said trunk busy.

5. In a telephone system, a two way trunk line connecting two exchanges,a relay in one exchange normally energized over said trunk line tomaintain the same in idle condition, means effective when the trunk lineis taken for use at the other exchange for deenergizing said relay, anda circuit closed by said relay to make said trunk line busy in the firstexchange.

6. In a telephone system, a two way trunk line, means at one end of saidtrunk line responsive to normal current flow thereover for maintainingsaid trunk line in idle condition, and means for reversing the currentflow in said trunk line when the same is taken for use at the other endfor making the trunk line busy.

7. In a telephone system, a two way trunk line connecting two exchanges,a polar relay in one exchange normally energized over said trunk line tomake said trunk line idle.

and means for reversing the direction of current flow in said trunk linewhen the same is taken for use at the other exchange, the said relaybeing responsive to such current reversal to make the said trunk linebusy. I

8. In a telephone system, a two way trunk line connecting two exchanges,means in one exchange for maintaining a current flow over said trunkline in one direction when the trunk line is idle and in the oppositedirection when the trunk line is in use, and

sive to current flow over said trunk line for maintaining the same idleor busy under the conditions named. respectively. y

9. In a telephone system, a two way trunk line connecting a manualexchange with an automatic exchange, electromagnetic means energizedresponsive to normal current How in said trunk line for rendering thesame accessible to switches in the automatic, exchange, and means foraltering the current flow when said trunk line is taken for use at themanual exchange whereby said first means is operated to make said trunkline busy to said switches.

10. In a telephone system, a two way trunk line connecting a manualexchange with an automatic exchange, a polar relav responsive to normalcurrent flow in said trunk line for rendering the same accessible toswitches in the automatic exchange, and means for reversing the currentflow when the trunk line is taken for use at the manual exchange, thesaid polar relay being responsive to such current reversal to render thesaid trunk line busy to said switches.

11. In a telephone system, a two way trunk line connecting twoexchanges, means in one exchange for maintaining a current flow oversaid trunk line in one direction when the trunk line is idle and in theopposite direction when the trunk line is in use, automatic switches inthe other exchange for connecting with said trunk line, and a polarrelay responsive to current flow over said trunk line for rendering saidtrunk line accessible to said switches when idle and busy to saidswitches when in use.

12. In a telephone system, a two way trunk line connecting twoexchanges, means in one exchange for maintaining a current flow oversaid trunk line in one direction when the trunk line idle and in theopposite direction when the trunk line is in use, automatic switches inthe other exchange for connecting with said trunk line and provided withtesting means for determining whether the trunk line is busy or idle,and a polar relay responsive to current flow over said trunk line in onedirection for causing the trunk line to test idle, and responsive tocurrent flow in the opposite direction for causing the trunk line totest busy.

19 In a telephone system. a trunk line, a signal for indicating when acall is received over said trunk line, a relay controlling said signal,a circuit for said relay, a short circuit around said relay to. preventthe same from energizing, and means responsive to ringing currentreceived over said trunk line for opening said short circuit to permitsaid relay to operate.

14. In a telephone system, a trunk line, a signal at one end of saidtrunk line for indicating when a call is received thereover, a relaycontrolling said signal, a circuit for said relay, a short circuitaround said relay to prevent the same from energizing,

means for closing said circuits when the trunk line is taken for use atthe other end, and means responsive to ringing current received oversaid trunk line for opening said short circuit to permit said relay tooperate.

15. In a telephone system, a trunk line connecting a manual exchangewith an automatic exchange, a high resistance bridged across said trunkline at the automatic exchange, means for setting up connections oversaid trunk line in both directions, means for short circuiting saidresistance whenever a connection is established, a line relay operatedresponsive to the short circuiting of said resistance when the calloriginates in the automatic exchange, and a supervisory relay responsiveto the short circuiting of said resistance when the call originates inthe manual exchange.

16. In a telephone system, a trunk line connecting twoexchanges, a linerelay and battery bridged across said trunk line at the first exchange,a high resistance normally bridged across said trunk line at the secondexchange, means for short circuiting said resistance when the trunk lineis taken for use at the second exchange to operate said line relay tosignal an operator at the first exchange, means for extending aconnection over said trunk line from a calling subscriber in the firstexchange to a called subscriber in the second exchange, and means forshort circuiting said resistance when the called subscriber answers inorder to give the operator in the first exchange a supervisory signal. 717. In a telephone system, a trunk line connecting a main exchange witha private automatic exchange and terminating at the latter point in anautomatic switch, said switch having automatic off normal releasemechanism, means for establishing a control circuit for said switch whenthe trunk line is taken for use at the main exchange, means foroperating said switch to connect with a subscribers line, and means forthen establishing the control circuit over a new path to retain theswitch in position independent of the main exchange connection or theconnection to said subscribers line.

18. In a telephone system, a trunk line connecting a main exchange witha private automatic exchange and terminating at the latter po nt in anautomatic switch, means for establishing a control circuit for saidswitch when the trunk line is taken for use at the main exchange, meansfor operating said switch to connect with a subscriber-s line, means forthen establishing the control circuit over a new path to retain theswitch in position independent of the main exchange connection or theconnection to said subscribers line, and a resistance included in thenew control circuit to reduce the flow of current therein.

19. In a telephone system, a trunk line connecting a main exchange witha branch exchange and terminating at the latter point in an automaticswitch, other switches in the branch exchange having access to saidtrunk line for establishing branch to main exchange connections, meansfor operating said first switch to connect said 7 in an automaticswitch, other switches in the branch exchange having access to saidtrunk line for establishing branch to main exchange connections, meansfor operating said first switch to connect said trunk line with asubscribers line in the branch exchange, means for retaining said switchin position independent of subscriber or main exchange control, andmeans responsive to the removal of the receiver on said subscribers linefor making said trunk line busy to the said other switches.

21. In a telephone system, a two-way trunk circuit, means includingautomatic switching mechanism for completing a connection to a calledsubscribers line over said trunk circuit in one direction, multipleterminals for said trunk circuit accessible to automatic switchesthrough the medium -of which a calling subscriber can set up aconnection over said trunk circuit in the opposite direction, and arelay. bridged across said trunk circuit and arranged to feed talkingcurrent to the called subscriber in the first connection and to thecalling subscriber in the second connection.

22. In a telephone system, a trunk line connecting a manual exchan ewith an automatic exchange and terminating at the latter exchange in anautomatic progressively movable switch, a repeating coil dividing saidtrunk line into two inductively connected sections, an operatorsswitchboard in the automatic exchange to which the second section ofsaid trunk line is multiply connected, relay mechanism actuated over thefirst section when the trunk line is taken for use at the manualexchange to close a control circuit for said switch over a conductorseparate from the talking circuit, and calling mechanism at saidswitchboard for interrupting said control circuit in accordance withdigits in a subscribers telephone number to operate said switch.

23. In a telephone system, a trunk line and test terminal therefor,automatic switches having access to said trunk line, means for placing apotential on said terminal to make the trunk line busy to said switches,a polar relay controlling the application of said busy potential, andmeans for reversing the direction of current flow in said trunk line tocontrol said polar re- 24. In a telephone system, a trunk line,automatic switches having access to said trunk line, multiple testterminals for said trunk line in the banks of said switches, a polarrelay having an armature selectively responsive to currents ofdifi'erent polarities flowing over said trunk line, a source of testpotential, and circuit arrangements controlled by said armature forcontrolling the application of said potential to said test terminals.

25. In a telephone system, a two wa trunk line, means at one end of saidtrun line responsive to normal current flow thereover for maintainingsaid trunk line in idle condition, an automatic switch for connectingwith said trunk line at the same end, and means for reversing thecurrent flow in said trunk line when the same is taken for use at theother end for making the trunk line busy to said switch.

26. In a telephone system, a two way trunk line connecting twoexchanges, a polar relay in one exchange normally energized over saidtrunk line to make said trunkline idle, an automatic switch in said oneexchange for connecting with said trunk line, and means forreversing'the direction of current flow in said trunk line when the sameis taken for use at the other exchange, the said relay being responsiveto such current reversal to "make the said trunk line busy to saidswitch.

27. In a telephone system a two way trunk line connecting two exc anges,means in one exchange for maintaining a current flow over said trunkline in one direction when the trunk line is idle and in the oppositedirection when the trunk line is in use, said means comprising a linerelay and an operators cord circuit together with suitable batteryconnections, automatic switches in the second exchange, and a polarrelay in the second exchange responsive to current flow over said trunkline for maintaining the same idle or busy to said switches under theconditions named, respectively.

28. In a telephone system, a trunk line terminating at one end in anautomatic switch, a polar relay normally energized over said trunk line,means for reversing the direction of current flow in said trunk linewhen the same is taken for use at the other end, and an operatingcircuit for said switch ing the direction of current flow in said,

trunk line when the same is taken for use at the first exchange, and acontrol circuit for said switch closed under control of said polar relayresponsive to such current reversal.

30. In a telephone system, a trunk line, an automatic switch forextendin a connection over said trunk line in one irection, a signal forcalling an operator to extend a connection over said trunk line in theother direction, a polar relay normall energized over said trunk line,means or o erating said polar relay by reversing t c direction ofcurrent flow over said trunk line to make the trunk line busy to saidswitch, and a circuit for said call signal closed under control of saidpolar relay re-.

sponsive to ringing current received over said trunk line.

31. In a telephone system, a trunk line, an automatic switch forextending said trunk line to a called line, an operators call siglnal, apolar relay bridged across said trunk e and normally energized, acontrol circuit for said switch closed under control.

of said relay responsive to a reversal of current in said trunk hue, anda circuit for said call signal closed under control of said relayresponsive to ringing current received over said trunk line. Y

32. In a telephone system, a two way trunk line, two switches associatedwith said trunk line at the same end thereof, one switch for extending aconnection to said trunk line and the other for extending a connectionfrom said trunk line, a polar relay normally energized over said trunkline, and means for operating said relay by reversing the direction ofcurrent flow in said trunk line to make the trunk line busy to oneswitch and close a control circuit for the other switch.

33. In a telephone system, a trunk line connecting two exchanges, meanswhereby an operator in the first exchange can signal an operatorin thesecond exchange, a relay at the second exchange energized by theoperator at the second exchange when she answers the call, meansincluding an automatic switch for extending the call to a line in thesecond exchange, a battery feed relay in the second exchange forsupplying talking current to the called station, a supervisory signal atthe first exchange, a circuit including a conductor of said trunk linefor controlling said signal, said circuit including a resistance at thesecond exchange, contacts on both said relays for short circuiting saidresistance to control said signal, and a locking circuit for the saidfirst mentioned relay controlled by said battery feed relay.

Signed by me at Chicago, Cook County Illinois this 18thday of Se tember1920.

fiERNARD DARWI WILLIS.

